Pre-heat oven to 350 degrees F.

Chop your veggies.

Bring a pot of water to boil and blanch broccoli, cauliflower, and brussels sprouts until the broccoli turns vibrantly green and all three veggies are somewhere between al-dente and fork-tender. This will take a few minutes, but no longer than 8. When your perfect texture is reached, shock the veggies in an ice bath (more info here) to halt the cooking process. No mushy veggies here! Alternatively if you'd prefer softer veggies, feel free to let them cook a smidge longer and/or skip the ice bath.

Place veggies in a casserole dish and set aside.

Drizzle a medium pot (one deep enough to make your cheese sauce in) with a little bit of olive oil or butter and caramelize your onions, stirring occasionally. Continue cooking until onions are golden brown and tender.

Turn your burner to medium-low heat and add 1/4 cup (4 TBSP) butter.

Once your butter has melted, slowly mix in flour a tablespoon at a time, whisking the mixture together as you go.

Once your roux has begun to form a paste with the onions, add room temperature (or slightly warmed) milk and cream, whisking constantly until the sauce thickens.

Remove your pot from heat (it will begin to thicken even further at this part) and slowly add 1/2 the cheese.

Right about now you'll want to dive in face first with a crusty baguette. I'll look the other way if you do, but try to save the sauce for the veggies, will ya!?

Pour sauce over veggies, then top with the cheese you set aside earlier.

Next melt 1 TBSP of butter in a small skillet and add breadcrumbs. Mix constantly while toasting, then sprinkle over casserole.

Bake on center rack at 350 for 20 mins.

Prepare to swoon.